1. Field of the Invention
The present invention relates to an electrode foil for a solid electrolytic capacitor, comprising a formed aluminum foil and having an improved voltage resistance and heat resistance at the cut-end part of the foil. The present invention also relates to a manufacturing method thereof and a solid electrolytic capacitor using the electrode foil.
2. Description of Related Art
Solid electrolytic capacitors use an electrode foil obtained by forming a dielectric layer comprising an anodic oxide film on the surface of an aluminum foil. This electrode foil is commonly manufactured by dipping an aluminum foil in an acidic electrolytic solution to effect a forming treatment on the surface thereof by an anodic oxidation process using the aluminum foil as an anode pole, and then cutting the foil into a predetermined size. In recent years, down-sizing of electrolytic capacitors for the low-voltage use is proceeding in the field of electronic equipments, and to cope with this, a high electrostatic capacitance per the area is demanded. Accordingly, in the production of an electrode foil for the low-voltage use, a forming treatment has been conventionally performed, wherein, as shown in FIG. 3, an aluminum foil is subjected to a surface roughening treatment by AC etching the foil in an electrolytic solution containing, for example, chloride ions to form a large number of pores 2 having a pore diameter of, for example, about 0.2 .mu.m on the surface thereof, so that the effective surface area of the aluminum foil can be enlarged. Thereafter, a thin and dense, uniform barrier film (anodic oxide film) 3 having few defects and a thickness of approximately from 0.01 to 0.1 .mu.m is formed on each surface of the aluminum foil, including inner surfaces of these pores. In the thus-obtained formed foil, a large number of pores 2 are formed on both surfaces of a core part 1 comprising an aluminum core metal, and on each surface having an effective area enlarged by the pores, the dense, uniform barrier film 3 having few defects is formed as a dielectric layer.
The formed foil obtained is cut into a size predetermined for respective small solid electrolytic capacitors for the low-voltage use, and then used as an electrode foil. At this time, the aluminum core metal is exposed at the cut-end part 5 of the electrode foil and the electrode cannot be integrated as it is into a solid electrolytic capacitor, but must be insulated. This cut-end part 5 is usually insulated by again subjecting the formed foil cut out to a forming treatment (hereinafter referred to as a "cut-end-forming") to form a barrier film on this cut-end part 5. The above-described formed foil is usually cut using the shearing force of a knife edge and accordingly, the aluminum core metal is exposed and made pointed at the cut-end part 5 to form a pinnacle part 6. If the cut-end part in this state is subjected to a cut-end-forming to form a barrier film on the cut-end part 5, heat is locally generated as a result of concentration of the electric current due to the pinnacle shape during voltage loading, and the barrier film may be ruptured by the thermal stress or other disadvantageous results are caused. Thus, the voltage resistance and heat resistance of the solid electrolytic capacitor is extremely reduced. Furthermore, if a surface roughening treatment or a cut-end-forming at a high voltage is carried out in the same manner as described above, in the case that the dielectric layer at the cut-end part 5 is reinforced, the barrier film 3 which has been already formed on each surface of the formed foil may be ruptured in either case.
Therefore, a method of insulating the cut-end part by covering it with a resin or the like, but without the formation of an anodic oxide film, has been proposed (see, for example, examined Japanese Utility Model Publication, No. Hei 6-14465). According to this method, however, the resin layer extends also to the surface of the electrode foil to thereby reduce the electrostatic capacitance the capacitor, and this controverts the requirement for down-sizing. Furthermore, the process is complicated and in turn, the production cost increases.
The present invention has been created to overcome the above-described problems, and accordingly, the object of the present invention is to provide a method for manufacturing an inexpensive electrode foil for a solid electrolytic capacitor by forming a voltage-resistant and heat-resistant film only on the cut-end part without causing any reduction in the effective area of the electrode foil, as well as an electrode foil for a solid electrolytic capacitor manufactured by this method, and a solid electrolytic capacitor using this electrode foil.